A steel wire is generally manufactured through a step of wire drawing a steel wire rod manufactured by hot rolling to a required wire diameter. In the wire drawing, it is necessary to sufficiently remove the scale deposited on the surface of the wire rod at the stage prior to processing in order to ensure favorable drawability.
The removal of such a scale has been mainly accomplished by acid pickling in the prior art. However, the acid pickling may unfavorably deteriorate the working environment, and further entails the disposal of liquid wastes after use. For these reasons, “mechanical descaling” (mechanical scale removal) for mechanically removing the scale has become performed in place of the acid pickling step.
The mechanical descaling is carried out not only through the process based on shot blast or air blasting, but also through the process in which the scale is peeled off by bending or twisting. On the other hand, if the scale is peeled off during transfer of a wire rod, the base metal is exposed, so that rust may form. Accordingly, there is a demand for the formation of such a scale as to be less likely to be peeled off during transfer, and more likely to be peeled off upon mechanical descaling for a steel wire rod after hot rolling.
In response to such a demand, as described in, for example, Japanese Laid-Open Patent Publication Nos. Hei 7-204726, 8-295992, 10-204582, and 11-172332, the following methods are adopted: the composition of the scale is controlled; the interface roughness between the base metal portion and the scale is controlled; and the thickness of the scale is controlled; and other methods.
However, in these prior arts, there is no philosophy that the Si concentration in the scale is controlled for enhancing the mechanical descalability. In addition, although the Si concentration in the scale depends upon the cooling rate after hot rolling in wire rod manufacturing, no close study has been made on the cooling conditions. As a result, although the methods pertain to a steel wire rod which has a scale with an appropriate peelability on the surface, they do not produce sufficient effects.